The present invention relates generally to motor drive systems and, more particularly, to a system and method for precharging components of a filter arranged to reduce harmonics injected onto AC power lines when operating a motor drive unit.
Often, power delivered from a power source or supply is not properly conditioned for consumption. For example, power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids that are designed to be extremely efficient at delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as fixed frequency three-phase alternating current (AC) power. As such, the power must typically be converted or “conditioned” prior to consumption.
For example, motors and their associated loads are one type of common inductive load employed at many consuming facilities that require power conditioning. When a motor is the consuming point, power “conditioning” systems are utilized to convert the fixed frequency AC power delivered over utility grids to a form suitable for driving the motor. To this end, power conditioning for motor systems typically include AC-to-DC (direct current) rectifiers that convert the utility AC power to DC power applied to positive and negative DC buses (i.e. across a DC link). The power distributed across the DC buses is then converted, for example, by use of an inverter, to AC power designed to drive the motor.
Specifically, referring to FIG. 1, the present invention will be described with respect to a motor system 10. The motor system 10 generally includes a power supply 12, a motor drive unit 14, and a motor 16. The power supply 12 provides power to the motor drive unit 14 that, in turn, converts the power to a more usable form for the motor 16 that drives an associated load 18.
The motor drive unit 14 includes a variety of components, such as a rectifier 20, an inverter 22, and a controller 24. During operation, the power supply 12 provides three-phase AC power, for example, as received from a utility grid over transmission power lines 26. However, it is also contemplated that the power supply 12 may deliver single-phase power. The rectifier 20 is designed to receive the AC power from the power supply 12 and convert the AC power to DC power that is delivered to positive and negative DC buses 28, 30 of a DC link 32. Specifically, the rectifier 20 includes a plurality of switches (e.g., diodes or silicon controlled rectifiers) to rectify the AC power received from the AC power lines 26.
The DC power is delivered from the rectifier 20 over the positive and negative DC buses 28, 30 to the inverter 22. The inverter 22 includes a plurality of switching devices (e.g., IGBTs or other semiconductor switches) that are positioned between the positive and negative buses 28, 30 and controlled by the controller 24 to open and close specific combinations of the switches to sequentially generate pulses on each of the supply lines 34 to drive the motor 16 and, in turn, the load 18 through a drive shaft 36.
While use of the rectifier 20 is an efficient means for converting the AC power received from the power lines 26 to DC power, harmonic currents made up of interger multiples of the power line frequency will be injected onto the power lines 26 as a result. These injected harmonic currents can adversely affect operation of the motor drive unit 14 and other systems connected to the power supply 12.
As a result, a filter 38 is often arranged between the power supply 12 and the motor drive unit 14. Typically, a passive filter 38 is arranged to suppress the harmonic currents injected onto the power lines 26 by operation of the rectifier 20 of the motor drive unit 14. The filter 38 often includes at least a first stage reactor 39 and a number of capacitors 40 that, at least partially, forms a second stage 41. When the motor drive unit 14 is stopped or operating at a low load, the passive filter 38 draws a significant leading power factor (pf) current due to the inclusion of capacitors 40 contained within the filter 38. Hence, the filter 38 acts as a leading pf load to the power supply 12. This can be particularly problematic if the power supply 12 includes backup generators that are ill equipped to handle a leading pf load.
One solution to this problem is to monitor the operation of the motor drive unit 14 and use a relay 42 to control operation of the capacitors 40. The relay 42 is configured to control a plurality of switches 44 to disconnect the capacitors 40 in the filter 38 when the motor drive unit 14 is stopped or operating below a predetermined speed or load. However, when reconnecting the capacitors 40 into the filter 38, the motor drive unit 14 may experience an overvoltage fault. Such overvoltage faults can generally be avoided if the voltage rating of the motor drive unit 14 is greater than the rated line voltage, for example, using a motor drive unit 14 rated for 480V on a 400V system. However, such over-rating can significantly raise the cost of the system.
It would, therefore, be desirable to have a system and method for filtering harmonics without drawing a leading pf current under some conditions or causing an overvoltage fault in other conditions and without the need to use over-rated components.